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[2003:cb:c713:d600:afc5:4312:176f:3fb0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441c0dfc537sm1328385e9.16.2025.05.02.08.11.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 May 2025 08:11:11 -0700 (PDT) Message-ID: Date: Fri, 2 May 2025 17:11:08 +0200 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 08/13] KVM: guest_memfd: Allow host to map guest_memfd() pages To: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org Cc: pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com References: <20250430165655.605595-1-tabba@google.com> <20250430165655.605595-9-tabba@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.04.25 18:56, Fuad Tabba wrote: > Add support for mmap() and fault() for guest_memfd backed memory > in the host for VMs that support in-place conversion between > shared and private. To that end, this patch adds the ability to > check whether the VM type supports in-place conversion, and only > allows mapping its memory if that's the case. > > This patch introduces the configuration option KVM_GMEM_SHARED_MEM, > which enables support for in-place shared memory. > > It also introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which > indicates that the host can create VMs that support shared memory. > Supporting shared memory implies that memory can be mapped when shared > with the host. > > Signed-off-by: Fuad Tabba > --- > include/linux/kvm_host.h | 15 ++++++- > include/uapi/linux/kvm.h | 1 + > virt/kvm/Kconfig | 5 +++ > virt/kvm/guest_memfd.c | 92 ++++++++++++++++++++++++++++++++++++++++ > virt/kvm/kvm_main.c | 4 ++ > 5 files changed, 116 insertions(+), 1 deletion(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 9419fb99f7c2..f3af6bff3232 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -729,6 +729,17 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm) > } > #endif > > +/* > + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for > + * private memory is enabled and it supports in-place shared/private conversion. > + */ > +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) > +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm) > +{ > + return false; > +} > +#endif > + > #ifndef kvm_arch_has_readonly_mem > static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm) > { > @@ -2516,7 +2527,9 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) > > static inline bool kvm_mem_from_gmem(struct kvm *kvm, gfn_t gfn) > { > - /* For now, only private memory gets consumed from guest_memfd. */ > + if (kvm_arch_gmem_supports_shared_mem(kvm)) > + return true; After our discussion yesterday, am I correct that we will not be querying the KVM capability, but instead the "SHARED_TRACKING" (or however that flag is called) on the underlying guest_memfd instead? I assume the function would then look something like if (!kvm_supports_gmem(kvm)) return false; if (kvm_arch_gmem_supports_shared_mem(kvm)) return .. TBD, test the gmem flag for the slot via gfn return kvm_mem_is_private(kvm, gfn); > + > return kvm_mem_is_private(kvm, gfn); > } > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index b6ae8ad8934b..8bc8046c7f3a 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -930,6 +930,7 @@ struct kvm_enable_cap { > #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237 > #define KVM_CAP_X86_GUEST_MODE 238 > #define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239 > +#define KVM_CAP_GMEM_SHARED_MEM 240 > > struct kvm_irq_routing_irqchip { > __u32 irqchip; > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig > index 559c93ad90be..f4e469a62a60 100644 > --- a/virt/kvm/Kconfig > +++ b/virt/kvm/Kconfig > @@ -128,3 +128,8 @@ config HAVE_KVM_ARCH_GMEM_PREPARE > config HAVE_KVM_ARCH_GMEM_INVALIDATE > bool > depends on KVM_GMEM > + > +config KVM_GMEM_SHARED_MEM > + select KVM_GMEM > + bool > + prompt "Enables in-place shared memory for guest_memfd" > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 6db515833f61..8bc8fc991d58 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -312,7 +312,99 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn) > return gfn - slot->base_gfn + slot->gmem.pgoff; > } > > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM > +/* > + * Returns true if the folio is shared with the host and the guest. > + */ > +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index) > +{ > + struct kvm_gmem *gmem = file->private_data; > + > + /* For now, VMs that support shared memory share all their memory. */ > + return kvm_arch_gmem_supports_shared_mem(gmem->kvm); Similar here: likely we want to check the guest_memfd flag. -- Cheers, David / dhildenb